A plug-in connection with the aforementioned features is described in DE 10 2004 035 354 B3. The known plug-in connection is based on the locking principle, in which on the one hand a resiliently expandable securing ring is arranged in a chamber formed in a union nut tightly screwed onto the threaded-connection body joined with a first pipe or forming a constituent thereof, and on the other hand a pipe piece in the form of a second pipe with a bead formed thereon by deforming its pipe wall can be inserted into the chamber of the union nut or adjoining threaded-connection body, and locked therein by means of the securing ring. During insertion of the pipe piece, the bead expands the resiliently designed securing ring, until the latter catches behind the locking surface formed by the bead upon reaching the connecting position of the threaded-connection body and pipe piece. To enable this radial movement by the securing ring, the floor surface of the chamber allocated to the securing ring has formed in it a stepped recess for accommodating the securing ring, which consists of a first step situated adjacent to the chamber and set up to accommodate the securing ring during insertion of the pipe piece, and a second step adjoining the latter that secures the securing ring so as to abut against the inserted pipe piece and lock it in place. Due to the allocation of these two steps of the recess, the known plug-in connection does not provide for a rotation of the union nut on the threaded-connection body with the pipe piece inserted.
Another disadvantage associated with such a plug-in connection is that the securing ring abuts against the transitional corner between the first recess step and second recess step in the initially unloaded inserted position of the pipe piece in the threaded-connection body. Only when the plug-in connection is exposed to pressure or placed under tensile load the securing ring is drawn completely into the second recess step, and fixed in place therein by the position of the union nut on the threaded-connection body. As a result, a certain axial backlash of the pipe piece with the locking surface is present relative to the threaded-connection body with union nut given variable pressures, and in particular given an unpressurized plug-in connection. Given exposure to a dynamic pressure or vibrations acting on the plug-in connection, this axial backlash leads to corresponding axial movements of the components relative to each other, and hence to wear, resulting in a lower service life for the plug-in connection.